In order to determine the bone density of a patient, there are currently two serious methods in the prior art. On the one hand dual energy x-ray absoptiometry (DEXA or DXA), on the other hand quantitative computed tomography (QCT).
With DEXA, two projective recordings of a patient are created using different x-ray energy spectra. On account of the different energy-specific absorption values for bone mineral and soft tissue, the portion of soft tissue can be subtracted and the proportion of the surface occupied by bone material can be determined. Measured values in g/cm2 bone material are obtained as the result.
By using the two DEXA projection recordings with different x-ray energy spectra, a material breakdown into three material compositions can also be performed so that a rough composition of the entire body can be determined. Fat mass, fat-free mass without bones and bone mass are generally determined here as material compositions.
During the QCT, projections are recorded of a patient from a plurality of projection directions with an x-ray source rotating about the patient, the projections presenting two different x-ray spectra. Two volume image data records of different x-ray energy spectra are reconstructed with these projections. A material breakdown can in turn be executed with these two volume image data records, so that a volume recording is available at the end which exclusively represents the existing bone material in a 3D representation. It is thus possible to determine the actual mass density of bone mineral in the bone and to specify the bone density as a specific density in kg/cm3.
The problem now is that the values determined in the DEXA and QCT for the bone density and also the recordings generated in each instance during these examinations are not directly comparable.